Not only did I commission and take an active role in a full restoration, I also had the hood and fender worked on in 2017, and finally had my hard top restored and the door replaced in 2019-2021. During the initial restoration, the restorer (primarily a body guy) was given a directive: if it is rusted, cut it out and weld in new metal. If it needs replacement, replace it. If you think it is rusted or think it needs replacement, that means it does and do it. As one who has been through it all, I'd offer that it's all in a matter of perspective.
Did you ever hear the old joke about the two shoe salesmen, dispatched from a London shoemaker to colonial Africa many years ago, to expand the maker's market? After a month in country, they were asked to report back to their boss. One of them says, "Bollocks! The outlook is all to pot, I'm afraid. Nobody wears shoes!" The other salesman says "I'm chuffed! The outlook is brilliant! Nobody wears shoes!" So you see, it's all a matter of perspective. I view a mechanically distressed car, or one missing a lot of parts with the same eye as I do rust. Both take time and money to repair.
Yes, absolutely, rust is precisely like an iceberg: it never shows completely. But if that were some secret, then we'd all be dismayed regularly. But it's no mystery and no secret, and a good body person knows how deep to go and how to solve it. There are cars sufficiently compromised in body that one should walk away from; and the same holds true for mechanically compromised.
Once the rust was fixed on my car, it's been a non-issue for the past 20 years. I suspect Pat Price who suggests rust repairs are not finite, based upon description of HIS rust repairs, has indeed solved them. Perhaps with more involvement than he'd like, and a lot more money, but solved nonetheless. I don't think he'll have to worry about it for a very long time, if ever.
Mechanical repairs don't end. As soon as you start that rebuilt engine it starts to wear. Ask AlexD about his motor mounts; just a mere SEVEN YEARS since replacement, and they need it replacement again. It took me a good 5-6 years to sort out the runnability of my car. Nothing ever seemed 100%. Was it fuel? Distributor? FIP adjustment? Wires? Spark? Coil? Tires? Why, YES! All of it. Look at all these posts here on the forum; how many discuss or lament continued body work on an ongoing basis? Not many. How many are trying to solve some kind of mechanical issue? Most. All the time.
Of course a rust free car--if there is such a thing--is preferable to one with known rust issues. But a car that is well sorted mechanically is a good find.